This invention relates to a material for use in a heat dissipation substrate for a semiconductor in the fields of IC, microwaves, and optics and, in particular, to a heat dissipation substrate for mounting a semiconductor device, a heat dissipation member used in a ceramic package encapsulating a semiconductor and a metal package similarly encapsulating a semiconductor, and a method of producing the same.
Traditionally, a heat dissipation member for use in applications of the type is required to have an excellent heat conductivity and to have a coefficient of thermal expansion approximate to that of alumina (Al2O3), beryllia (BeO), or aluminum nitride (AlN) which is a main constituent material of the semiconductor or the package
For the applications of the type, use has presently been made of a composite alloy obtained by sintering a compact of tungsten powder in a hydrogen atmosphere to produce a porous tungsten (W) material and impregnating the material with copper (Cu).
In recent years, the semiconductor is operated at a higher frequency and increased in capacity. This brings about a situation where the copper-tungsten composite alloy limited in heat conductivity is insufficient. Specifically, in case of a ceramic package using alumina as an insulator, the package is assembled by bonding alumina and a heat dissipation substrate by a silver brazing alloy. However, in order that the coefficient of thermal expansion of the composite alloy has a value approximate to that of alumina in a temperature range between normal temperature and about 780° C. at which the silver brazing alloy is solidified, the ratio of copper in the copper-tungsten composite must be suppressed between 10 and 13%. Therefore, limitation is imposed upon the thermal conductivity.
This is because the thermal conductivity of the composite is determined by its composition. If any defect such as a void is not present in the material and if constituent metals do not make a solid solution so that no alloy is produced, the thermal conductivity is determined by the ratio of the constituent metals. However, if a metal making a solid solution with the constituent metals is added, the thermal conductivity is decreased.
In case of the copper-tungsten composite alloy used as the heat dissipation substrate of the ceramic package encapsulating the semiconductor, a very small amount of an iron-group metal such as nickel (Ni) is generally added. The addition of the iron-group metal is applied in order to improve the wettability and to facilitate infiltration of copper into a void or gap in the porous tungsten material. By the above-mentioned addition, the thermal conductivity is decreased as compared with the binary composite of copper and tungsten.
On the other hand, in case of a combination of molybdenum (Mo) and copper, addition of any other metal is unnecessary because melted copper is excellent in wettability to molybdenum. In addition, since molybdenum and copper make no substantial solid solution, the thermal conductivity of a composite thereof is determined by a volumetric ratio therebetween.
In the meanwhile, the present inventors have already proposed a composite which is obtained by press-molding molybdenum powder to produce a powder compact and impregnating the powder compact with copper and which is excellent in thermal conductivity and suitable as a heat dissipation substrate for a semiconductor used in a large-capacity inverter or the like (see Japanese Patent Application No. 9-226361, hereinafter called a prior art 1).
The composite obtained by the prior art 1 is good in rollability. It has also been proposed that a heat dissipation substrate of a greater size is obtained by a rolling process.
Recently, a large-capacity semiconductor device accompanied with generation of a large amount of heat is used in an increased number of applications. One example is an inverter of an automobile energized by electricity as a driving force. In this case, it is necessary to convert electric power of several tens watts. When a semiconductor device having a rectifying function is driven, a large amount of heat is generated. In order to release the heat through a radiator to the outside of a car system, use is generally made of a following structure.
A rectifying device is mounted on an insulator substrate (such as AlN). A plurality of similar insulator substrates are fixed and attached to a large-sized heat dissipation substrate by soldering. The heat dissipation substrate is fixed to the radiator by screws or the like. The heat dissipation substrate is required to have an excellent heat conductivity and to have a heat expansion characteristic such that deformation resulting from a difference in coefficient of thermal expansion during cooling after soldering of the insulator substrates is suppressed small. Furthermore, the heat dissipation substrate is required to have a sufficient strength to allow the substrate to be fixed to the radiator by the screws or the like.
For the above-mentioned application, the present inventors have proposed a composite material of molybdenum and copper, which is manufactured without taking into account a rolling rate.
In view of energy saving in automobiles, there arises a demand for a heat dissipation substrate having a light weight in addition to the above-mentioned thermal characteristics. The light weight can be achieved by reducing the thickness of the heat dissipation substrate.
However, if the thickness of the heat dissipation substrate is reduced, the heat capacity is decreased. In addition, the deformation resulting from thermal strain due to the difference in coefficient of thermal expansion in case where the insulator substrates are soldered is increased as compared with the case where the thickness is great. The deformation is a hindrance to the contact between the substrate and the radiator and prevents transfer of the heat.
Thus, it is required to provide a material which is excellent in thermal conduction as compared with the composite material of molybdenum and copper according to the prior art 1 and which has a low coefficient of thermal expansion in a range such that occurrence of the problems related to the thermal strain upon soldering of the insulator substrates can be prevented.
For the above-mentioned application, AlN excellent in thermal conduction is generally used as the insulator substrate to be soldered to the heat dissipation substrate. During cooling after soldering the insulator substrate to the heat dissipation substrate, there arise problems, such as deformation of the heat dissipation substrate and fracture of the insulator substrate, as a result of the thermal strain. In order to prevent the occurrence of the above-mentioned problems, the material of the heat dissipation substrate is required to have a coefficient of thermal expansion of 9.0×10−6/K or less at a temperature not higher than 400° C. This is because, if the material having a coefficient higher than 9.0×10−6/K is used and if the heat dissipation substrate is soldered to ceramic such as AlN, deformation may be caused or cracks may be produced in a bonded portion or the ceramic itself during heat shrinkage.
On the other hand, apart from the application to the inverter of the electric automobile mentioned above, a ceramic package is used to mount a semiconductor device for producing microwaves in the field of communication or the like. In such ceramic package also, a heat dissipation substrate having following characteristics in addition to excellent thermal conduction is required in order to release heat produced by the semiconductor device to the outside of the package.
As the ceramic for the ceramic package, use is generally made of a material containing Al2O3 as a main component. For the heat dissipation substrate, it is required to use a material such that, in case where the substrate is bonded to the ceramic by a high-temperature (about 800° C.) brazing material (CuAg eutictic brazing material or the like), the ceramic is not broken and the heat dissipation substrate is less deformed during cooling after brazing due to the thermal strain resulting from the difference in coefficient of thermal expansion from the ceramic.
In particular, in the event that the semiconductor device, such as GaAs, which produces high-temperature heat during operation and which is poor in thermal conduction is used, it is strongly desired to use a material excellent in thermal conduction at its surface to be contacted with the device. For this purpose, the Cu—W composite material generally used and the Mo—Cu composite material according to the prior art 1 may be insufficient in thermal conduction.
At present, use is sometimes made of a [Cu/Mo/Cu] clad material (hereinafter called CMC) in order to satisfy the above-mentioned requirement. However, the CMC clad material is disadvantageous in the following respects.
In the CMC clad material, a Cu layer as each surface layer is softened around a brazing temperature (800° C.) and is easily deformed during cooling. The clad material exhibits a thermal behavior similar to that of Mo. Therefore, as compared with the ceramic (generally containing Al2O3 as a main component) to be bonded, heat shrinkage is small so that the CMC composite is deformed. When the package is attached to a cooling device by screws or the like, the deformation prevents sufficient contact with the cooling device. Thus, there is a problem in cooling of the semiconductor.
Consideration will be made about mechanical characteristics of the substrate. Mo as an intermediate layer of the CMC clad material is brittle. Therefore, if a substrate part is punched out by a press from a plate material, cracks tend to occur in the Mo layer. In particular, the above-mentioned clad material has the soft Cu layers on both sides thereof. Therefore, it is difficult to prevent occurrence of the cracks in the Mo layer during punching. In view of the above, the substrate part must be produced by electric spark machining which generally requires high machining cost.
On the other hand, Cu—W and Cu—Mo generally used as the heat dissipation substrate for the semiconductor ceramic package are typically bonded by the silver brazing alloy. Since W and Mo are poor in wettability with the silver brazing material, the surface of the Cu—W or Cu—Mo substrate is subjected to Ni plating. Thus, brazing with the ceramic subjected to metallization requires a Ni plating process for the substrate. In addition, various problems, such as blister, stain, and discoloration, will be caused due to insufficient contact of a Ni plating layer. Thus, there is a problem in yield or reliability.
In view of the above, it is a first object of this invention to provide a method of producing a semiconductor-mounting heat dissipation substrate which is for use as a heat dissipation substrate of a ceramic package and which is superior in thermal conductivity to a CMC clad material and easy in machining by a punch press.
It is a second object of this invention to provide a method of producing the above-mentioned semiconductor-mounting heat dissipation substrate.
It is a third object of this invention to provide a semiconductor-mounting heat dissipation substrate of a copper-clad type, which has a thermal expansion characteristic such that various problems resulting from thermal strain are not caused even if it is brazed with ceramic.
It is a fourth object of this invention to provide a method of producing the above-mentioned semiconductor-mounting heat dissipation substrate of a copper-cladding type.
It is a fifth object of this invention to provide a ceramic package using the above-mentioned semiconductor-mounting heat dissipation substrate of a copper-clad type.
It is a sixth object of this invention to provide a method of producing the above-mentioned ceramic package.